A film capacitor, which is a type of capacitor, includes flexible resin films serving as a dielectric and metal films serving as first and second counter electrodes, the resin films being interposed between the metal films that facing each other, and the metal films being arranged on both main surfaces of the resin films. The film capacitor usually has a cylindrical shape obtained by winding the dielectric resin films. First and second external terminal electrodes are formed on opposing first and second end faces of the cylinder. The first counter electrode is electrically connected to the first external terminal electrode. The second counter electrode is electrically connected to the second external terminal electrode.
As the dielectric resin film for a film capacitor, for example, Japanese Unexamined Patent Application Publication No. 10-119127 (PTL 1) describes a biaxially oriented polypropylene film in which the molecular structure (atacticity) and the amount of an antioxidant added are optimized, and also describes a film capacitor including the biaxially oriented polypropylene film as a dielectric resin film, the film capacitor having excellent heat resistance and breakdown voltage. Usually, the upper limit of the operating temperature of polypropylene used as a material for a dielectric resin film of a film capacitor is 85° C. However, the film described in PTL 1 has an improved breakdown voltage, i.e., 540 V/μm at 105° C., and thus can be used at a higher temperature.
However, the guaranteed temperature is improved as described above in the film of PTL 1 but is only up to 105° C. The heat resistance is therefore not sufficient. That is, the film is not suitable for high temperature applications, for example, use in engine compartments of automobiles.
In the case of producing a film from a thermoplastic resin, such as polypropylene, usually, a pellet material is melted, extruded, and drawn. However, it is very difficult to produce a film having a thickness of less than 2.5 μm. Thus, it is not easy to reduce the thickness of a dielectric resin film to miniaturize the film capacitor.
For example, International Publication No. WO2006/100833 (PTL 2) discloses the use of polyisocyanate-crosslinked polyvinyl acetal as a material for a dielectric resin film in place of polypropylene, and that a dielectric filler is added in order to improve a dielectric constant. The polyvinyl acetal can be dissolved in a solvent. Accordingly, unlike polypropylene, a thin-layer sheet can be formed without using thermal melting or drawing.
However, when an electric field is applied to the dielectric resin film, the electric field is applied only to a resin portion of the film because the dielectric resin film disclosed in PTL 2 contains the filler having a high dielectric constant. As a result, the resin portion is likely to be damaged, thereby reducing the breakdown voltage. That is, a direct electric field is applied to the resin portion having a high volume resistivity in the dielectric resin film. In the case of an AC electric field, the AC electric field is mainly applied to the resin portion having a low dielectric constant.